Civil engineering structures can be exposed to various loads such as traffic, earthquakes and wind. Monitoring structural response, such as displacement or deflections, to such dynamic loads can play an important role in assessing the ongoing structural health of the structures and preventing structural failure.
Contact-type sensors, such as linear variable differential transformers, can be employed for measuring structural displacements. However, this can necessitate installation and cabling between the measurement point of the structure and a stationary platform, which can be difficult, and/or impracticable, for a large-size structure in the field. Accelerometers, which do not require a stationary reference point, can also be used to measure displacements through double integration of acceleration, but numerical errors can arise as a concern, and sensor installation and cabling can also be problematic for such systems.
Displacement monitoring by video image processing with so-called vision-based displacement sensors can be used as an alternative. Such systems can employ a digital camcorder and a personal computer to remotely monitor structural displacement. Certain systems, however, have required installation of a target on the structure of interest having a high contrast black-and-white pattern. Such installation can be costly and burdensome, and the targets typically require weather protection for long-term monitoring.
Likewise, certain vision-based displacement sensors are also susceptible to measurement error caused by ambient or environmental displacement of the camera system due, for example, to ground vibrations caused by traffic or weather conditions. These concerns increase with increasing distance of the vision-based displacement sensor from the structure being monitored and the corresponding magnification of the image of the structure being monitored.
Accordingly, a need exists for an accurate low-cost vision displacement sensor that is easily installed and which tracks the displacement of existing features of a structure.